Visible-infrared plane grating imaging spectrometer

ABSTRACT

An imaging spectrometer, covering the visible through infrared wavelengths, which disperses the light by a plane diffraction grating behind a wedged optical element. This design uses an achromatic doublet lens with a reflective coating on its convex back surface to produce the spectra on a flat detector. Spatial keystone distortion and spectral smile are controlled to less than one tenth of a pixel over the full wavelength range, facilitating the use of simple retrieval algorithms.

This application claims priority to provisional application Ser. No.61/847238 filed Jul. 17, 2013, the contents of which are incorporatedherein by reference.

This invention was made with government support under Contract Na.FA8721-05-C-0002 awarded by the U.S. Air Force. The government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of Endeavor

The present invention relates to an imaging spectrometer, and more to anarrangement of optical elements to provide full spectral imaging over awide waveband, in this case covering the visible through infrared in acompact instrument.

2. State of Technology

The challenge in imaging spectrometers is to make them small enough tofit in the small unmanned aerial vehicles which are now available. Forremote sensing purposes, these aerial vehicles present a savings inoperating costs. Small. imaging spectrometers are also advantageous forman-portable instruments and for fitting into existing industrialproduction lines. Currently, reflective imaging spectrometers withconvex gratings are typically used in the visible-infrared region.Making the convex gratings with low straylight for thesevisible-infrared spectrometers is difficult and expensive. Reflectiveimaging spectrometers are also too large for the smaller unmanned aerialvehicles.

U.S. Pat. No. 5,880,834 issued to Michael P. Chrisp Mar. 19, 1999provides the following state of technology information: “There are threeproblems in designing an imaging spectrometer where light in a slice ofan image field passing through an entrance slit is to be diffracted by agrating parallel to the slit and imaged onto a focal plane for displayor recording with good spatial resolution parallel to the slit and goodspectral resolution perpendicular to the slit: 1. Eliminatingastigmatism over the spectrum on the image plane. 2. Removing fieldcurvature from the spectrum focused onto the image plane. 3. Obtaininggood spatial resolution of the entrance slit which involves eliminatingastigmatism at different field angles from points on the entrance slit.”The disclosure of U.S. Pat. No. 5,880,834 is incorporated herein in itsentirety for all purposes.

There are two methods for the making the convex gratings for the aboveimaging spectrometer, electron beam lithography and single point diamondturning. The gratings are difficult to make because for good diffractionefficiency the blaze angle needs to be constant to the local surfacetangent, U.S. Pat. No. 6,480,333 discloses an electron beam fabricationmethod developed at the California Institute of technology. Thesegratings are extremely good, but having only one facility in the US haslimited their application. Nowadays, most imaging spectrometers withconvex gratings have them fabricated by single point diamond turning.Unfortunately these gratings tend to have high scattered light in thevisible. Diamond turned mirror surfaces, when used in the visible aretypically post polished to reduce the surface roughness, but with agrating surface this is impossible without destroying the grooves, sothe intrinsic surface roughness is that generated by the diamond turningprocess.

A number of catadioptric designs have been developed for the infrared.However, these designs are based on two properties of infrared opticalmaterials, high refractive index and low spectral dispersion, so theyare not suitable of the visible-infrared spectral region.

U.S. Pat. No. 7,414,719 for an imaging spectrometer wide fieldcatadioptric design issued to Michael P. Chrisp Aug. 19, 2008 providesthe following state of technology information: “The present inventionprovides a compact imaging spectrometer with an immersive or reflectivediffraction grating that compensates optical distortions. The imagingspectrometer comprises an entrance slit for transmitting light, a systemwith a catadioptric lens and a dioptric lens for receiving the light anddirecting the light, an immersion grating, and a detector array. Theentrance slit, the system for receiving the light, the immersiongrating, and the detector array are positioned wherein the entrance slittransmits light to the system for receiving the light and the system forreceiving the light directs the light to the immersion grating and theimmersion grating receives the light and directs the light through thesystem for receiving the light to the detector array. The compactimaging spectrometer uses smaller cryogenic coolers facilitating itsusing in portable (man carried) gas detection systems and in smallunmanned aerial. vehicles for remote gas detection.” The disclosure ofU.S. Pat. No. 7,414,719 is incorporated herein in its entirety for allpurposes.

SUMMARY OF THE INVENTION

Features and advantages of the present invention will become apparentfrom the following description. Applicants are providing thisdescription, which includes drawings and examples of specificembodiments, to give a broad representation of the invention. Variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this descriptionand by practice of the invention. The scope of the invention is notintended to be limited to the particular forms disclosed and theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention is an imaging spectrometer that covers the visiblethrough infrared, dispersing the light with a plane diffraction gratingbehind a wedged optical element. This compact design uses an achromaticdoublet lens with a reflective coating on its convex back surface toproduce the spectra on a flat detector. The invention includes anentrance slit for transmitting light, an achromatic lens with areflective back surface, a plane reflective diffraction grating immersedin a wedged optical element, an order sorting filter, and avisible-infrared detector. The plane grating, which can be replicated,cut, ruled or etched, is easier to fabricate than the convex gratingrequired for comparable imaging spectrometers.

The powers and surface shapes of in the invention are chosen to correctthe Petzval curvature, the chromatic aberration, the spatial keystonedistortion and the spectral smile. The spatial keystone distortion iscontrolled to less than one tenth of a pixel over the full wavelengthrange from the visible through infrared, and the spectral smile is keptto less than one tenth of a pixel over the wavelength range. The lowspatial distortion means that the spectral information from a pixel onthe ground is dispersed over a single column of pixels, providingsimultaneous correlated spectral information without the need toresample.

In another embodiment of the visible-infrared plane grating imagingspectrometer constructed in accordance with the present invention, thereis a modification wherein the two lenses of the achromatic doublet areno longer cemented together but have been separated. With no opticalcement this embodiment can be used in the mid-wave and long-waveinfrared and the ultraviolet region.

In yet another embodiment of the visible-infrared plane grating imagingspectrometer constructed in accordance with the present invention, thereis a modification wherein the reflective coating has been removed fromthe back of the achromatic doublet and its function implemented by aseparate concave mirror.

In yet another embodiment of the visible-infrared plane grating imagingspectrometer constructed in accordance with the present invention, thereis a modification wherein the plane reflective diffraction gratingimmersed in the wedged optical element has been replaced by a separatewedged optical element and a separate plane reflective diffractiongrating which is no longer immersed.

In yet another embodiment of the visible-infrared plane grating imagingspectrometer constructed in accordance with the present invention, thereis a modification wherein the wedged optical element has been removedfrom the system so the plane reflective diffraction grating operates inair.

In yet another embodiment of the visible-infrared plane grating imagingspectrometer constructed in accordance with the present invention, thereis a modification wherein the achromatic lens with a reflective backsurface has been replaced by a triplet achromatic or apochromatic lenswith a reflective back surface.

The invention is susceptible to modifications and alternative forms.Specific embodiments are shown by way of example. It is to be understoodthat the invention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are incorporated into and constitute apart of the specification, illustrate specific embodiments of theinvention and, together with the general description of the inventiongiven above, and the detailed description of the specific embodiments,serve to explain the principles of the invention.

FIG. 1 is a schematic illustration of an embodiment of thevisible-infrared plane grating imaging spectrometer constructed inaccordance with the present invention, with the visible through infraredbeing dispersed and focused on the relevant parts of the detector withspatial resolution along the slit.

FIG. 2 is a perspective view of the embodiment of the visible infraredimaging spectrometer.

FIG. 3 is another embodiment of the visible-infrared plane gratingimaging spectrometer constructed in accordance with the presentinvention illustrated in FIG. 1 that is a modification wherein the twolenses of the achromatic doublet are no longer cemented together buthave been separated.

FIG. 4 is another embodiment of the visible-infrared plane gratingimaging spectrometer constructed in accordance with the presentinvention illustrated in FIG. 1 that is a modification wherein thereflective coating has been removed from the back of the achromaticdoublet and its function implemented by a separate concave mirror.

FIG. 5 is another embodiment of the visible-infrared plane gratingimaging spectrometer constructed in accordance with the presentinvention illustrated in FIG. 1 that is a modification wherein the planereflective diffraction grating immersed in the wedged optical elementhas been replaced by a separate wedged optical element and a separateplane reflective diffraction grating which is no longer immersed.

FIG. 6 is another embodiment of the visible-infrared plane gratingimaging spectrometer constructed in accordance with the presentinvention illustrated in FIG. 1 that is a modification wherein thewedged optical element has been removed from the system so the planereflective diffraction grating operates in air.

FIG. 7 is another embodiment of the visible-infrared plane gratingimaging spectrometer constructed in accordance with the presentinvention illustrated in FIG. 1 that is a modification wherein theachromatic lens with a reflective back surface has been replaced by atriplet achromatic or apochromatic lens with reflective back surface.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This visible-infrared plane grating imaging spectrometer was developedfor remote sensing for smaller scale unmanned aerial vehicles. Withfewer optical elements to assemble, and an easily fabricated planegrating, it is easier to fabricate and assemble than the reflectiveoptical systems utilizing convex gratings. Use of conventionalreplicated gratings leads to lower stray light than the diamond turnedvisible gratings with their high surface roughness grooves. The spatialkeystone and spectral smile have been controlled over the fullwavelength range, to ensure the purity of the spectral data receivedfrom the ground footprints which facilitates the accuracy of thespectral matching algorithms.

Referring now to FIG. 1 and FIG. 2, one particular embodiment is shownof a visible-infrared plane grating imaging spectrometer constructed inaccordance with the present invention with the visible through infraredbeing dispersed and focused onto the detector. The grating rulings areparallel to the X axis resulting in the light spectrally dispersed alongthe Y axis, with the spatial information resolved for the differentwavelengths along the X axis.

The light travels from the entrance slit 101 of the spectrometer to thefirst lens 102 of the cemented achromatic doublet, and onto the secondlens 103, the reflective coating on the curved back 104 of the lensreturns the light. The light then traverses back through the doubletlens 103 and 102, and travels on through the wedged refractive element105 to be diffracted off the immersed reflective plane grating 106. Thespectrally dispersed light now passes back through the wedged refractiveelement 105 and on through the lower portion of the doublet lens 102 and103, the reflective coating on the curved back 104 of the lens returnsthe light, back thorough 103 and 102 which focus the light onto thedetector array 108 after passing through the order sorting filter 107,with spectrum spread between the long wavelength end 110 and the shortwavelength end 111. The order sorting stepped spectral handpass filter107, enables rejection of unwanted spectral orders when the spectralregion covers multiple octaves. For spectral regions less than an octavethe order sorting filter 107 is not necessary.

The defining aperture stop 109 for the spectrometer a the entrance ofthe wedged optical element 105 helps reduce the stray light in thesystem.

The reflective diffraction grating 106 has the rulings immersed inoptical contact with the wedged refractive optical element 105. Thereflective diffraction grating can he formed by a conventional gratingoptically cemented to the wedged refractive optical element 105, or byreplicating a grating in an adhesive layer on the surface of the wedgedoptical element 105 with a reflective coating applied. Alternately bysuitable material choice the diffraction grating 106 could he etched ordiamond cut into the surface of the wedged optical element 105, with areflective coating applied. The design uses a conventional grating 106with equally spaced straight grooves on a flat surface. The wedge angleof the optical element 105 is used to control distortion and the itsrefractive surface shape can compensate for aberrations, with power forreducing the Petzval curvature and aspheric terms for correcting thespherical and somatic aberrations.

Referring to FIG. 2 the overall size of the optical system is 65 mm inthe Y dimension, 65 mm in the X dimension and 70 mm in the Z dimension.If the unused sides of the lens are removed, the size of the system is65 mm in the Y dimension, 40 mm in the X dimension and 70 rum in the Zdimension.

In the design the optical powers of the achromatic lens element 102,103and its curved reflective surface 104, together with any power on thewedged optical element 105 are adjusted to remove the Petzval curvaturefrom the optical design, enabling the dispersed spectrum to be focusedonto the flat visible-infrared detector 108. The opposite sign of thePetzval curvature from the concave reflecting surface balances thePetzval curvature of the positive lens and any residual curvaturebalanced by the power of the wedged optical element if necessary.

In the design the spectral smile and spatial keystone distortion arecorrected by the wedge angle and surface shape of the wedged optical 105element and the surface shapes and powers and geometry of the achromaticlens 102,103,104.

TABLE 1 Optical Design Parameters Parameter Value wavelength range 0.4to 2.5 microns f-number at slit and detector 3 input pupil approximatelytelecentric detector format 640 × 240 pixel size 30 microns Performanceover full detector area Optical performance 80 percent encircled energy<30 microns Spatial mapping distortion: <0.1 pixel change in spatialmapping with wavelength (keystone) Spectral distortion; spectral <0.1pixel smile Ghost level <0.1% of primary image

Table 1 shows that this refractive design meets the challengingrequirements over this wavelength range. The spatial keystone distortionhas been kept to less than 0.1 pixel over the full wavelength range,enabling very good spectral correlation of the pixels at allwavelengths. This means that the spectrum from a single point in theentrance slit wanders from the center of a column of pixels by less than±1.5 microns.

The spectral smile, which is the curvature of the slit image at eachwavelength on the detector, has been kept to less than 0.1 pixel.Correcting the spectral slit curvature, and the spatial mappingdistortion with wavelength, to less than one tenth of a pixel ensuresthat the images do not have to be resampled to correct for thesedistortions.

The image quality, given by the eighty percent encircled energy, is goodover the full wavelength range, so the spectral line spread function ismainly determined by the entrance slit width and the pixel size. Sincethe spectrum covers multiple octaves, a cooled order sorting filter 107in front of the detector prevents contamination from the unwantedspectral orders and reduces the infrared thermal background. The filtercan either have different sections with the different spectralbandpasses or can be a linear variable filter.

Referring now to FIG. 3, another embodiment of the visible-infraredplane grating imaging spectrometer constructed in accordance with thepresent invention illustrated in FIG. 1, that is a modification whereinthe two lenses of the achromatic doublet are no longer cemented togetherbut have been separated. The airspaced achromatic doublet consists oftwo separate lenses, a singlet lens 302, and a singlet lens 303 with areflective coating 304 on its back surface. This lens has more degreesof freedom for aberration control having an additional optical surface,plus aspherics can now be used on the two inner surfaces. With thegrating etched or cut into the wedged optical element, optical cementcan be avoided altogether along with its limited spectral transmission.With suitable optical material choice this embodiment can be used in themid-wave infrared, the long wave infrared or the ultraviolet.

The light travels from the entrance slit 301 of the spectrometer to thefirst lens 302 of the air spaced achromatic doublet, onto the secondlens 303, the reflective coating on the curved back 304 of the lensreturns the light. The light then traverses back through the two lenses303 and 302, and travels on through the wedged refractive element 305 tobe diffracted off the immersed reflective plane diffraction grating 306.The spectrally dispersed light now passes back through the wedgedrefractive element 305 and on through the lower portion of the twolenses 302 and 303, the reflective coating on the curved back 304 of thelens returns the light, passing back thorough 303 and 302 and focusedonto the detector array 308, after passing through the order sortingfilter 307.

Referring now to FIG. 4, another embodiment of the visible-infraredplane grating imaging spectrometer constructed in accordance with thepresent invention illustrated in FIG. 1, that is a modification whereinthe reflective coating has been removed from the back of the achromaticdoublet and its function implemented by a separate concave mirror 404.This enables the separate adjustment of the power and aspheric surfacesfor the mirror 404 and the hack of the lens 403.

The light travels from the entrance slit 401 of the spectrometer to thefirst lens 402 of the cemented achromatic doublet lens and passesthrough the second lens 403 to be reflected back from the concave mirror404. The light then traverses back through the achromatic doublet lens103 and 102, and then travels on through the wedged refractive element405 to be diffracted off the immersed reflective plane grating 406. Thespectrally dispersed light now passes back through the wedged refractiveelement 405 and on through the lower portion of the achromatic doubletlens 402 and 403, to be reflected off the concave mirror 404, travelingback thorough 403 and 402 which focus the light onto the detector array408 after passing through the order sorting filter 407.

Referring now to FIG. 5, another embodiment of the visible-infraredplane grating imaging spectrometer constructed in accordance with thepresent invention illustrated in FIG. 1, that is a modification whereinthe plane reflective diffraction grating immersed in the wedged opticalelement has been replaced by a separate wedged optical element 505 and aseparate plane reflective diffraction grating 506 which is no longerimmersed. This enables the shape of both sides of the separate wedgeoptical element 505 to be optimized for correcting the aberrations andreduces the complexity of immersing the grating.

The light travels from the entrance slit 501 of the spectrometer to thefirst lens 502 of the cemented achromatic doublet, onto the second lens503, and the reflective coating on the curved back 504 of the lensreturns the light. The light then traverses back through the doubletlens 503 and 502, and travels on through the separate wedged refractiveelement 505 to be diffracted off the reflective plane diffractiongrating in air 506. The spectrally dispersed light now passes backthrough the separate wedged refractive element 505 and on through thelower portion of the doublet lens 502 and 503, and the reflectivecoating on the curved back 504 of the lens returns the light, backthrough 503 and 502, which focus the light onto the detector array 508after passing through the order sorting filter 507.

Referring now to FIG. 6, another embodiment of the visible-infraredplane grating imaging spectrometer constructed in accordance with thepresent invention illustrated in FIG. 1, that is a modification whereinthe wedged optical element has been removed from the system so the planereflective diffraction grating 605 operates in air. This is asimplification of the design for when the spatial and spectraldistortion requirements can be relaxed.

The light travels from the entrance slit 601 of the spectrometer to thefirst lens 602 of the cemented achromatic doublet, onto the second lens603, and the reflective coating on the curved back 604 of the lens thenreturns the light. The light then traverses back through the doubletlens 603 and 602, and travels on to be diffracted off the reflectiveplane diffraction grating in air 605. The reflected spectrally dispersedlight now passes back through the lower portion of the doublet lens 602and 603, and the reflective coating on the curved back 604 of the lensreturns the light. The light then travels back thorough 603 and 602which focus the light onto the detector array 607 after passing throughthe order sorting filter 606.

Referring now to FIG. 7, another embodiment of the visible-infraredplane grating imaging spectrometer constructed in accordance with thepresent invention illustrated is FIG. 1, that is a modification whereinthe achromatic lens with a reflective back surface has been replaced bya triplet achromatic or apochromatic lens with reflective back surface702, 703, 704, 705. The triplet lens enables better secondary colorcorrection, maintaining the performance for faster f-numbers or forwider spectral regions, or for more challenging regions such as theultraviolet.

The light travels from the entrance slit 701 of the spectrometer to thefirst lens 702 of the cemented triple, on through the second lens 703and third lens 704, with the reflective coating on the curved back 705of the lens returning the light. The light then traverses back throughthe triplet lens 704, 703, 702, and then travels on through the wedgedrefractive element 706 to be diffracted off the immersed reflectiveplane grating 707. The spectrally dispersed light now passes backthrough the wedged refractive element 706 and on through the lowerportion of the triplet lens 702, 703, 704, and the reflective coating onthe curved back surface 705 of the lens returns the light, travelingback thorough 704, 703, 702 which focuses the light onto the detectorarray 709 after passing through the order sorting filter 708.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims. This invention can be adapted to different spectralregions by suitable choice of refractive and reflective materials, andwith different detector arrays. This application covers allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the following appended claims.

What is claimed is:
 1. A visible-infrared plane grating imaging spectrometer, comprising: an entrance slit for transmitting light, an achromatic lens with a reflective back surface, a plane reflective diffraction grating immersed in a wedged optical element, an order sorting filter, a visible-infrared detector; said entrance slit, said achromatic lens with a reflective back surface, said plane reflective diffraction grating immersed in a wedged optical element, said order sorting filter, and said detector for visible and infrared radiation positioned wherein, said entrance slit transmits light to said achromatic lens with reflective back surface, which refracts light and then reflects the light from the back surface, passing back through the lens and is directed to said plane reflective diffraction grating immersed in a wedged optical element, said wedged optical element refracts the light then the reflective diffraction grating spectrally disperses and directs the light back through wedged optical element, which refracts light to said achromatic lens with a reflective back surface, said achromatic lens with a reflective back surface refracts light and then reflects the light from the back surface, passing hack through the lens and focuses it to said order sorting filter, said order sorting filter transmits the selected spectral orders to the said visible-infrared detector.
 2. The visible-infrared plane grating imaging spectrometer of claim 1 wherein the optical powers of the said achromatic lens element with and the power of its curved reflective surface, together with the power on the wedged optical element are adjusted to remove the Petzval curvature from the optical design and to ensure that the dispersed spectrum is focused onto the flat said visible-infrared detector.
 3. The visible-infrared plane grating imaging spectrometer of claim 1 wherein the wedge angle and surface shape of the wedged element in the said plane reflective diffraction grating immersed in a wedged optical element, and the surface shapes and powers of the achromatic lens and its curves reflective surface are adjusted to compensate for the spectral smile and keystone distortion
 4. The visible-infrared plane grating imaging spectrometer of claim 1 wherein the spacing and curvature of the groove spacing of the said plane reflective diffraction grating immersed in a wedged optical element, is varied to correct the spectrally varying aberrations and distortion, said reflective diffraction grating optical cemented to the wedged optical element, replicated or directly cut or etched into the wedged optical element.
 5. The visible-infrared plane grating imaging spectrometer of claim 1 wherein the two lens elements of the said achromatic lens with a reflective back surface are no longer cemented, but separated into two separate lenses, with the second lens having the reflective back coating, the lens surface shapes between the lenses are varied independently and are aspheric for aberration correction.
 6. The visible-infrared plane grating imaging spectrometer of claim 1 wherein the reflective coating is removed from the said achromatic lens with a reflective back is replaced by a separate concave mirror after the lens, the light now passes through the achromatic lens is refracted by the back surface to the concave reflective mirror, which reflects the light back through the lens, the shapes of the mirror surface and the back surface of the achromatic lens are adjusted for aberration correction.
 7. The visible-infrared plane grating imaging spectrometer of claim 1 wherein said plane reflective diffraction grating immersed in a wedged optical element is split into two separate optical elements, a wedged refractive optical element and a plane reflective grating in air, light passes through the wedged refractive optical element, is spectrally dispersed the plane reflective grating and directed hack through the wedged refractive optical element.
 8. The visible-Infrared plane grating imaging spectrometer of claim 1 wherein said plane reflective diffraction grating immersed in a wedged optical element is replaced by a plane reflective diffraction grating in air, the powers of the refractive elements and the curved back surface of the said achromatic lens with reflective back surface are adjusted to reduce the Petzval curvature.
 9. The visible-infrared plane grating imaging spectrometer of claim 1 wherein said achromatic lens with a reflective back surface element is replaced by an achromatic or apochromatic triplet lens with a reflective back surface, wherein the optical powers of the said triplet lens and its curved reflective back surface, together with any power on the wedged optical element are adjusted to remove the Petzval curvature from the optical design and to ensure that the dispersed spectrum is focused onto the flat said visible-infrared detector. 